Genetic evidence for cooperative polyandry in reverse dichromatic Eclectus parrots

Group size in cooperatively polyandrous birds appears to be limited by the diminishing probability that any one male will achieve paternity as group size increases. Here we use molecular techniques to analyse the mating system of Eclectus parrots, a species with a surprisingly large number of males (up to seven) in each group. We show that their social groups are not composed of kin, and that all males seek direct fitness. Most broods of two nestlings had just one father. However, further group males gained paternity in other nesting attempts within and between breeding seasons. Although the likelihood of multiple paternity increased with the number of males in the group, only a subset of group males gained paternity over the eight-year study period. Some males gained paternity at irregular intervals with the same female over multiple years, and many also mated polygynandrously at widely dispersed nests. Our data also confirm that females monopolise scarce breeding hollows over multiple years. We suggest that Eclectus parrots occur in larger group sizes than other cooperatively polyandrous species as a result of a severely biased operational sex ratio brought about by a scarcity of breeding females and a male-biased adult sex ratio.